Severe or disabling pain is a frequent result from spinal cord injury (SCI). Pain often occurs within weeks or months of the spinal cord injury, and typically persists or even intensifies with time. Over the past half century, studies have illustrated that patients suffering a SCI suffer anywhere from between 20 and 77% incidence of some level of severe or disabling chronic pain. Davis et al., Clin Orthop 112:76-80 (1975); Richards et al., Pain 8:355-366 (1980); Siddall et al., Spinal Cord 39:63-73 (2001); Stormer et al., Spinal Cord 446-455 (1997); Woolsey, J Am Paraplegia 9:39-41 (1986). It has also been well documented that patients suffering from at least some level of severe or disabling chronic pain show reduced rehabilitation potential and tend to have a significant overall reduction in quality of life. As such, severe or disabling chronic pain, incident from SCI, is a major concern for patients with SCI, is a major concern within the health care industry, and is a major concern within society in general.
Patients suffering from SCI have pain(s) that originate in the central nervous system, often termed central pain, and includes pain defined as central deafferentation pain. Eide, Spinal Cord 36:601-612 (1998); Ishijima et al., Appl Neurophysiol 51:2-5, 175-187 (1988); Melzack et al., Pain 4:195-210 (1978); Yezierski, Pain 68:185-194 (1996). Central deafferentation pains are believed to involve the abnormal up-regulation of neuronal activity after nerve injury. Electrophysiological studies have suggested that SCI causes abnormal changes in the firing pattern of neurons which signal pain sensation, including increased spontaneous activity of the neurons, reduced neuron thresholds, increased responsiveness to peripheral stimulation (hyperexcitability), prolonged after-discharges to repetitive stimulation, and the expansion of the peripheral receptive fields of central neurons. Eide, Spinal Cord 36:601-612 (1998).
Central pain has proven notoriously difficult to treat, often proving recalcitrant to modern medical and surgical pain treatment procedures. Of particular therapeutic significance, is the surgical treatment of specific dorsal root entry zone(s) (DREZ(s)) of the spinal cord. Surgical treatment of a central pain generating DREZ at the level of an injury is believed to disrupt the neural, i.e., electrical, communication and/or generation of aberrant pain signals that result from the injury. Initially, empiric techniques have been used to target DREZ sites for surgical treatment, resulting in modest outcomes for the patient, i.e., DREZ sites at the site of injury targeted for treatment. Friedman et al., J Neurosurg 65:465-469 (1986); Ishijima et al., Appl Neurophysiol 51:2-5, 175-187 (1988); Rath et al., Acta Neurochir 138:4, 364-369 (1996); Rath et al., Sterotact Funct Neurosurg 68:1-4, Pt 1, 161-167 (1997). One of the more relevant patient studies using this empirical technique suggests that approximately 50% of patients so treated achieve good relief from SCI associated pain. Friedman et al., J Neurosurg 65:465-469 (1986). In that series, at-level pain, i.e., pain at the immediate vicinity of the injury, responded best (74% “good results”) and below-level pain, i.e., pain below the level of injury, responded poorly (20% “good results”). These results suggest that empiric DREZ lesioning techniques can provide satisfactory relief for about half the number of patients treated, especially if the patient is suffering from pain in the vicinity of the injury. However, the results also suggest that a significant number of patients do not receive benefit from the technique, especially where the patient is suffering from pain perceived below the level of the injury.
More recently, a single study on patients suffering from SCI found that when intramedullary recordings of spontaneous neuroelectric hyperactivity were used to direct DREZ lesioning, substantially better outcomes were attained. Edgar et al., J Spinal Dis 6:48-56 (1993). This more targeted DREZ identification technique provides evidence that neuroelectrical targeting techniques could provide better outcomes for eradicating SCI based pain. Against this backdrop the present invention has been developed.